Tube and DMOSFET electrostatic headphone amplifiers.

Here are schematics and simulations of 2 electrostatic amplifiers. These amplifiers were previously posted both on esldiy and on my website on December 26th. 2013

The first uses DMOSFETS throughout.

This second uses both DMOSFETS and 6SN7 tubes.

The credit for one unusual feature of these designs goes to John Broskie. By tying the feedback network from the output to the +ve input on the opamp, and thence to the NEGATIVE opamp rail, this network controls the quiescent DC voltage at the output; the ratio of the 2 resistors times the negative rail voltage giving the value of the quiescent voltage: 22*15=330V

@Wakibaki: First off, I honestly had never seen your schematic earlier. It was posted on websites I don't usually read. Had I seen it, I wouldn't have posted mine since yours is working better. I'd rather have contacted you about it. At the time of posting, I really thought that this particular mix of parts hadn't been done before. Clearly I was wrong.

@all: Secondly, I should probably have linked to the RKV which was my main source of inspiration. But since I had just posted a thread about it, with proper references, and that the posting of the schematic was, in my mind, just a continuation of it, I must admit I forgot. That much I'm willing to confess. Please accept my apologies for a moment of carelessness.

Then, to answer the last direct question, I didn't credit Broskie because the schematic I posted doesn't make use of his material. It is the biasing scheme of the RKV I used, not his. The one thing Wakibaki gives credit to Broskie for is exactly the one thing that makes our schematics quite different. Ironically, it is also the one thing that makes his superior.

When I wrote I started with Tubecad, it is literally that seeing that schematic made me think I could extend the "variations on the rkv theme" to electrostatic amps. Broskie's basic hybrid building block is just the same as the rkv (a tube controlled by an opamp wrt gain but also operating points, especially plate voltage). So I tried to see if I could do it somehow differently.

Finally, there aren't that many way to skin a cat. I don't pretend the schematic I posted is something revolutionary or even new; all the elements are well known if not trivial. There aren't many tubes able to sustain the voltages needed and even fewer high voltage depletion mosfet with low capacitance. The controversy between srpp and resistor loading is spread over so many threads, who am I supposed to credit ?

...will be cheaper, since there's no necessity for a heater supply. Since I've designed the boards to be 100*100mm, I can get them fabbed at minimum cost, this one is all thru-hole, so easy for diy assembly. I've got the case designed in Autocad for cutting on my CNC machine, all rectangular parts, aluminium and perspex, KISS. 5in*5in*4in deep. There's a digital readout for the bias voltage, that's the righthand pot. The readout uses an off-the-shelf ebay voltmeter. I have nearly all the parts including the FETs and 1250V WIMA caps. I just have to buy some transformers to be absolutely sure about the fit.

The layered design means that the PSU and its tracking is separated from the signal board by a ground plane. All the power drops down to where it's required on risers, completely orthogonal to the signal board. Standard PCB stand-offs tie the structure together.

Progress on the 'phones is slow, but steady. I bought the earpads made to fit off ebay.

The whole thing, amp and 'phones, could be supplied as a flatpack kit, including pretensioned Mylar diaphragms. The parts are all simple in the extreme, and could all be cut from sheet perspex with the addition of some nylon studding (threaded rods) and wingnuts. Oh, and I need a metal (aluminium) back for the amp to act as a heatsink. I can send them out for manufacture, it's a simple job on a CNC machine, and a lot of 10 wouldn't break the bank. They'd be the most accessible (cheapest) electrostatic 'phones + amp in existence. If that's what I decide to do. I kind of like not being a MOT, so I might just supply the boards at cost like I did with some of my other designs.

THD @ 20kHz for the all-DMOSFET version, 5mA quiescent per side, loaded each side with 1M||120pF. Should lose some even harmonics due to push-pull cancellation. All you can say is that THD will be lower than the 0.88% total shown here with ~500V pk-pk, probably in the region of 0.2%. You can probably get LTSpice to calculate the THD from the subtracted outputs, but I can't see a simple way, and TBH I don't care that much, these numbers are best used to pick between paper designs, not for forecasting an exact performance.